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Protein‑protein interaction
1. Pull‑down
The pull‑down assay is an effective in vitro technique for validating protein‑protein interactions, commonly used to confirm interacting proteins identified by yeast two‑hybrid systems or other screening methods. The fundamental principle involves immobilizing a target protein (the “bait”) onto a solid matrix. When cell lysates or other protein‑containing solutions are passed through the column, proteins that interact with the bait protein bind to the matrix, while non‑interacting contaminants flow through. The bound interacting proteins can then be eluted using specific buffers or conditions. Subsequent analysis typically involves SDS‑PAGE separation followed by Western blotting (to detect interactions between the bait and known proteins) or mass spectrometry (to identify novel interacting partners). This method is straightforward, easy to perform, and does not require hazardous radioactive materials, making it widely applicable in protein interaction analysis.
1.1 Technical Comparison
| Parameter | GST Pull‑down | His‑tag pull‑down | Streptavidin‑Biotin pull‑down |
|---|---|---|---|
| Tag size | ~26 kDa (Large) | ~0.8‑1 kDa (Minimal) | ~0.244 kDa (Biotin) |
| Binding ligand | Glutathione (GSH) | Ni²⁺/Co²⁺‑NTA Chelate Resin | Streptavidin |
| Binding affinity (kd) | ~10‑6 M (Moderate) | ~10‑13 M (High) | ~10‑14‑10‑15 M (Extremely High) |
| Binding specificity | High | Moderate (nonspecific binding possible) | Extremely High |
| Solubility enhancement | Significantly enhances solubility | No enhancement | No enhancement |
| Impact on protein structure | May alter folding and native conformation | Minimal, rarely affects function | Minimal, does not affect function |
| Elution conditions | Free glutathione (Mild) | Imidazole gradient or low pH | Free biotin or denaturing conditions |
| Denaturing conditions | Not applicable | Applicable (8 M urea) | Applicable |
| Expression system compatibility | E. coli (Optimal) | E. coli, Yeast, Insect, Mammalia | All expression systems |
| Detection sensitivity | Moderate | Moderate | Extremely high (low‑abundance proteins) |
| Background noise | Low | Higher (nonspecific binding) | Extremely low |
| Cost | Low | Low | Higher |
| Applications | Soluble expression of difficult proteins; Protein interaction domain mapping | Routine recombinant protein; purification Purification under denaturing conditions | Low‑abundance interaction detection; In vivo biotinylated protein capture |
| Key limitations | Large tag may interfere with function; Dimerization artifacts | Nonspecific binding risk; Buffer sensitivity | Difficult elution; Endogenous biotinylated protein interference |
1.2 GST Pull‑down
1.2.1 Introduction
GST pull‑down is a classical in vitro protein interaction verification technique based on the high‑affinity binding between Glutathione S‑Transferase (GST) and Glutathione (GSH). A target protein is fused with GST and expressed, then immobilized onto GSH‑coated magnetic beads or agarose resin to serve as the “bait”. Upon incubation with a lysate or purified protein containing candidate binding partners (the “prey”), specific interactions lead to capture and enrichment of the prey. After stringent washing to eliminate non‑specific binding, the interacting proteins are identified via Western blot or mass spectrometry.
1.2.2 Products
1.2.3 Applications
| Applications | Description |
|---|---|
| In vitro protein‑protein interaction validation | Utilizes GST fusion protein as “bait” to capture interacting “prey” proteins from cell lysates or purified protein preparations, validating known or predicted protein interactions |
| Interaction domain mapping | Determines the minimal functional domain mediating protein interaction through construction and analysis of a series of truncated mutants |
| Interaction affinity assessment | Semi‑quantitatively evaluates binding strength between proteins by performing pull‑down experiments with concentration gradients |
| Effect of post‑translational modifications on interactions | Analyzes the regulatory role of phosphorylation, acetylation, and other modifications on interactions by comparing pull‑down efficiency between wild‑type and modification site mutants |
| Small molecule compound screening | Employs target protein as bait to screen potential binding molecules or interaction disruptors from compound libraries |
| Protein complex component identification | Captures multi‑protein complexes associated with GST fusion proteins, combined with mass spectrometry analysis to identify complex constituent members |
References:
- Smith DB, Johnson KS. Single‑step purification of polypeptides expressed in Escherichia coli as fusions with glutathione S‑transferase. Gene. 1988 Jul 15;67(1):31‑40. doi: 10.1016/0378‑1119(88)90005‑4. PMID: 3047011.
- Hein MY, Hubner NC, Poser I, Cox J, Nagaraj N, Toyoda Y, Gak IA, Weisswange I, Mansfeld J, Buchholz F, Hyman AA, Mann M. A human interactome in three quantitative dimensions organized by stoichiometries and abundances. Cell. 2015 Oct 22;163(3):712‑23. doi: 10.1016/j.cell.2015.09.053. Epub 2015 Oct 22. PMID: 26496610.
- Huttlin EL, Ting L, Bruckner RJ, Gebreab F, Gygi MP, Szpyt J, Tam S, Zarraga G, Colby G, Baltier K, Dong R, Guarani V, Vaites LP, Ordureau A, Rad R, Erickson BK, Wühr M, Chick J, Zhai B, Kolippakkam D, Mintseris J, Obar RA, Harris T, Artavanis‑Tsakonas S, Sowa ME, De Camilli P, Paulo JA, Harper JW, Gygi SP. The BioPlex Network: A Systematic Exploration of the Human Interactome. Cell. 2015 Jul 16;162(2):425‑440. doi: 10.1016/j.cell.2015.06.043. PMID: 26186194; PMCID: PMC4617211.
- Wan C, Borgeson B, Phanse S, Tu F, Drew K, Clark G, Xiong X, Kagan O, Kwan J, Bezginov A, Chessman K, Pal S, Cromar G, Papoulas O, Ni Z, Boutz DR, Stoilova S, Havugimana PC, Guo X, Malty RH, Sarov M, Greenblatt J, Babu M, Derry WB, Tillier ER, Wallingford JB, Parkinson J, Marcotte EM, Emili A. Panorama of ancient metazoan macromolecular complexes. Nature. 2015 Sep 17;525(7569):339‑44. doi: 10.1038/nature14877. Epub 2015 Sep 7. PMID: 26344197; PMCID: PMC5036527.
1.3 His‑tag Pull‑down
1.3.1 Introduction
His‑tag pull‑down is an in vitro protein interaction technique based on Immobilized Metal Affinity Chromatography (IMAC), leveraging the high‑affinity coordination between a hexahistidine tag (His₆) fused to the N‑ or C‑terminus of a recombinant protein and transition metal ions (e.g., Ni²⁺, Co²⁺). The “bait” protein is immobilized onto Ni‑NTA (nickel‑nitrilotriacetic acid) or Co‑NTA magnetic beads/agarose resin. Upon incubation with cell lysates, tissue extracts, or in vitro translation products containing candidate “prey” proteins, specific capture occurs. After washing away non‑specific binders, the interaction complex is identified and analyzed.
1.3.2 Products
1.3.3 Applications
| Applications | Description |
|---|---|
| PPI validation | His‑tagged bait protein is immobilized on Ni‑NTA/TALON resin to capture interacting prey proteins from cell lysates or in vitro expression systems, validating known or screening novel protein interaction partners. |
| Protein complex purification and assembly analysis | Multi‑subunit protein complexes (e.g., SWI/SNF chromatin remodeling complex) are purified via His‑tag pull‑down to analyze complex composition, subunit interactions, and assembly mechanisms. |
| Ubiquitination studies | His‑tagged ubiquitin is expressed and ubiquitinated substrates are enriched under denaturing conditions via Ni‑NTA pull‑down to detect ubiquitination levels and chain types. |
| Protein‑DNA interaction | His‑tagged proteins are used to pull down biotinylated or fluorescently labeled DNA probes to study binding of transcription factors and chromatin remodeling proteins to specific DNA sequences. |
| Protein‑RNA interaction | His‑tag pull‑down is used to enrich protein‑RNA complexes for identifying RNA‑binding proteins and their target RNAs, or studying viral protein‑RNA interactions. |
References:
- Li Z, Burgos‑Bravo F, Xu K, Li C, Kwan KY, Tong AB, Shan Z, Wang H, Takaku M, Li J, Shi Z, Lyumkis D, Bustamante C, Fei J. Phase‑separated NDF‑FACT condensates facilitate transcription elongation on chromatin. Nat Cell Biol. 2025 Nov;27(11):1938‑1951. doi: 10.1038/s41556‑025‑01778‑8. Epub 2025 Sep 30. PMID: 41028835; PMCID: PMC12611769.
- Choe J, Lin S, Zhang W, Liu Q, Wang L, Ramirez‑Moya J, Du P, Kim W, Tang S, Sliz P, Santisteban P, George RE, Richards WG, Wong KK, Locker N, Slack FJ, Gregory RI. mRNA circularization by METTL3‑eIF3h enhances translation and promotes oncogenesis. Nature. 2018 Sep;561(7724):556‑560. doi: 10.1038/s41586‑018‑0538‑8. Epub 2018 Sep 19. PMID: 30232453; PMCID: PMC6234840.
- Tabana Y, Babu D, Fahlman R, Siraki AG, Barakat K. Target identification of small molecules: an overview of the current applications in drug discovery. BMC Biotechnol. 2023 Oct 10;23(1):44. doi: 10.1186/s12896‑023‑00815‑4. PMID: 37817108; PMCID: PMC10566111.
1.4 Streptavidin‑Biotin Pull‑down
1.4.1 Introduction
Streptavidin‑Biotin pull‑down is an in vitro target fishing technique built upon the ultra‑high affinity non‑covalent interaction between Streptavidin and Biotin. Biotin serves as a small‑molecule labeling probe, chemically conjugated to candidate drug molecules, metabolites, or ligands to form a “biotinylated bait”. After incubation with cell lysates, the bait is captured using streptavidin immobilized on agarose beads or magnetic beads. Given the extraordinary binding strength and stability, the complex remains intact even under stringent washing conditions (high salt, detergents, extreme pH), enabling efficient enrichment and identification of low‑abundance interacting proteins.
1.4.2 Products
1.4.3 Applications
| Applications | Description |
|---|---|
| PPI studies | Biotinylated bait protein is immobilized on streptavidin beads to capture interacting prey proteins from cell lysates, followed by MS or Western blot identification. |
| Protein complex separation and identification | Isolation of specific protein complexes from complex biological samples to characterize multi‑protein assembly composition. |
| Nucleic acid‑protein interaction studies | Biotinylated DNA/RNA probes capture DNA/RNA‑binding proteins (e.g., transcription factors, RBPs) for downstream analysis. |
| Post‑translational modification studies | Combined with proximity labeling to biotinylate and capture proteins in specific subcellular niches or modification states. |
| Drug target discovery and validation | Biotinylated small‑molecule compounds capture target proteins to validate mechanism of action in drug discovery. |
| Protein expression and purification | As an affinity tag system for efficient purification of recombinant proteins. |
References:
- Weber PC, Ohlendorf DH, Wendoloski JJ, Salemme FR. Structural origins of high‑affinity biotin binding to streptavidin. Science. 1989 Jan 6;243(4887):85‑8. doi: 10.1126/science.2911722. PMID: 2911722.
- Chodosh LA, Buratowski S. Purification of DNA‑binding proteins using biotin/streptavidin affinity systems. Curr Protoc Protein Sci. 2001 May;Chapter 9:Unit 9.7. doi: 10.1002/0471140864.ps0907s12. PMID: 18429216.
- Branon TC, Bosch JA, Sanchez AD, Udeshi ND, Svinkina T, Carr SA, Feldman JL, Perrimon N, Ting AY. Efficient proximity labeling in living cells and organisms with TurboID. Nat Biotechnol. 2018 Oct;36(9):880‑887. doi: 10.1038/nbt.4201. Epub 2018 Aug 20. Erratum in: Nat Biotechnol. 2020 Jan;38(1):108. doi: 10.1038/s41587‑019‑0355‑0. PMID: 30125270; PMCID: PMC6126969.
- Mimmi S, Zimbo AM, Rotundo S, Cione E, Nisticò N, Aloisio A, Maisano D, Tolomeo AM, Dattilo V, Lionello R, Fioravanti A, Di Loria A, Quirino A, Marascio N, Russo A, Trecarichi EM, Matera G, Quinto I, Torti C, Iaccino E. SARS CoV‑2 spike protein‑guided exosome isolation facilitates detection of potential miRNA biomarkers in COVID‑19 infections. Clin Chem Lab Med. 2023 Mar 27;61(8):1518‑1524. doi: 10.1515/cclm‑2022‑1286. PMID: 36972680.